diff --git a/index.js b/index.js
index e9d92d0..a23a75c 100644
--- a/index.js
+++ b/index.js
@@ -7,7 +7,7 @@ const url = require("url");
 const asExpression = require("as-expression");
 const dotty = require("dotty");
 const getJson = require("axios-get-json-response");
-const { ValidationError, validateValue, required, isString, arrayOf, allowExtraProperties } = require("validatem");
+const { ValidationError, validateValue, validateArguments, required, isString, arrayOf, allowExtraProperties } = require("validatem");
 
 let LookupFailed = createError("LookupFailed");
 let MethodNotAvailable = createError("MethodNotAvailable");
@@ -155,11 +155,15 @@ function attemptWellKnown(host) {
 
 module.exports = {
 	LookupFailed: LookupFailed,
-	discover: function (host) {
+	discover: function (hostname) {
+		validateArguments(arguments, [
+			["hostname", required, isString]
+		]);
+
 		return Promise.try(() => {
-			return attemptWellKnown(host);
+			return attemptWellKnown(hostname);
 		}).catch(MethodNotAvailable, () => {
-			return attemptLiteralHostname(host);
+			return attemptLiteralHostname(hostname);
 		}).catch(MethodNotAvailable, () => {
 			throwLookupError("none of the autodiscovery methods were available");
 		});